1. Field of the Invention
The present invention relates, in general, to a metallic interconnection material for solid oxide fuel cells which are operable at relatively high temperatures and, more particularly, to a metallic interconnection material for solid oxide fuel cells, which is superb in electric conductivity and oxidation resistance. Also, the present invention is concerned with a method for preparing such a metallic interconnection material.
2. Description of the Prior Art
In fuel cells, the interconnection material (ICM) has two important functions of electrically connecting one cell to another cell in a cell stack and separating feed gases so as not to mix them in a cell. In relation to this, the interconnection material is also termed "bipolar plate" or "separator".
To play a perfect role in the electric connection and the gas separation, the interconnection material is required to be chemically stable in oxidation and reduction environments and structurally dense enough to be gas-tight as well as show high electrical conductivity to allow electrons to continuously flow.
In particular, since solid oxide fuel cells are operated at a high temperature of 600-1,000.degree. C., their interconnection materials must be of chemical stability at such a high temperature and of thermal interchangeability with other components of cells in a range from room temperature to the operation temperature of the cells. Additionally, the requirements for the interconnection material include high mechanical strength, matching in thermal expansion with other cell components (especially, solid ceramic electrolytes) and low ion conductivity.
There have conventionally been used two kinds of interconnection materials: ceramic and metallic.
Representative of ceramic interconnection materials are LaCrO.sub.3 -based interconnection materials with a perovskite structure. The ceramic materials suffer from a disadvantage in that the reduction of Cr.sup.+4 into Cr.sup.+3 in a reducing atmosphere increases the ion radius, resulting in a volume expansion of the materials. In detail, since the opposite sides of a bipolar plate made of an LaCrO.sub.3 -based ceramic material are respectively operated under different oxygen partial pressures in a fuel cell, either a volume expansion takes place only on one side of the bipolar plate to bend the plate. Even if the plate is not bent by virtue of the load of the stack itself, the stress created within the material becomes larger than the breaking strength of the material, resulting in a breakage of the stack. Other disadvantages of the ceramic interconnection materials are that they are low in thermal conductivity, mechanical strength and electric conductivity and difficult to process and prepare.
Metallic interconnection materials, developed as a result of extensive efforts to solve the problems encountered in the ceramic interconnection materials, are superior in electrical conductivity, processability, thermal conductivity and mechanical strength to the ceramic interconnection materials, but disadvantageous in that they are high in coefficient of thermal expansion with vulnerability to oxidation. The metallic interconnection materials developed thus far are exemplified by Al.sub.2 O.sub.3 /Inconnel 600 cermet [see, H. Seto, T. Miyata, A. Tsunoda, T. Yoshida and S. Sakurada, Proceedings of the third International Symposium on Solid Oxide Fuel Cells, S. C. Singhal and H. Iwahara (eds.), The Electrochemical Society, Inc., NJ 08534-2896 p.421 (1993)], (LaSr)CoO.sub.3 coated with Ni--20Cr, and Y.sub.2 O.sub.3 - or La.sub.2 O.sub.3 -dispersed Cr alloys (see, U.S. Pat. No. 5,407,758).
Of the conventional metallic interconnection materials, Ni alloys are large in the coefficient of thermal expansion and show poor oxidation resistance at such a high temperature as is necessary for the operation of the fuel cells. As for Cr alloys, they have an advantage over the Ni alloys in the coefficient of thermal expansion, but the oxide layers formed on them disadvantageously are of high volatility at high temperatures. Furthermore, the Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 or La.sub.2 O.sub.3 added in the metal alloys is lacking in electric conductivity, giving rise to a great increase in the electric resistance of the oxide layers formed upon oxidation. The volatilization and electric conductivity reduction of the Cr oxide layers increases the internal resistance of the fuel cells and the polarization resistance of air poles, resulting in a significant decrease in the life span of the fuel cells.
Another metallic interconnection material is found in U.S. Pat. No. 5,407,758 which discloses a ducrolloy comprising a small quantity of iron and rare earth metals, identified as Cr5Fe1Y.sub.2 O.sub.3. This ducrolloy has a coefficient of thermal expansion similar to those of ceramic solid electrolytes and is excellent in electric conductivity compared with conventional interconnection materials while showing low surface oxidation even after use for a long period of time by virtue of the presence of Y.sub.2 O.sub.3. With these advantages, a great interest has been taken in the ducrolloy as an interconnection material for high temperature type fuel cells. However, there remains a need for an improvement for this chrome-based alloy material as it suffers from disadvantages in that the volatilization of chrome makes the air pole poor in performance and the Cr oxide scale formed on the metal surface has so low electric conductivity that an increase is brought about in the contact resistance between the air pole and the interconnection material.